I think all one has to do is tell them it is not working out and good bye. If you want to try to explain it further, you can but do not have to do so. I don't think of it as confrontational, but simply where you are at and what you are going to do. These t people work for you. You pay them for whatever it is they do. Clients quit or stop or leave for all kinds of reasons and the ts are not to get mad or try to stop you. Good luck with the appointment.